This invention relates to a method of producing a silicon carbide base sintered material containing boron as a sintering assistant.
Silicon carbide base ceramics are excellent in many aspects of physical and chemical properties including mechanical strength, heat resistance and corrosion resistance and accordingly can be used as the material of various structural elements.
In producing a silicon carbide base ceramic material of a desired shape by a sintering process, it is known to use boron as a sintering assistant. According to Japanese patent application primary publication No. 50-78609 (1975), for example, a silicon carbide base sintered material is produced by the steps of first mixing a powder of silicon carbide with a powder of boron and a phenolic resin and dispersing the powder mixture in an organic solvent by, e.g., ball milling to thereby obtain a uniformly mixed slurry in which the phenolic resin is dissolved, then removing the solvent from the slurry by a suitable drying method such as spray drying to obtain a powdery or granular mixture of the firstly named ingredients, forming that mixture into a desired shape by a suitable technique such as press compacting or injection molding, heating the shaped material in a nonoxidizing atmosphere until carbonization of the phenolic resin and finally raising the heating temperature up to 1800.degree.-2200.degree. C. to thereby accomplish sintering of silicon carbide in the presence of boron.
However, the products of such a sintering process often suffer from locally unsound sintering and, in extreme cases, there are voids as large as tens of microns in the sintered material. When the sintered material is under some load of stress it is likely that fracture of the sintered material starts at any one of such unsoundly sintered regions. From a practical point of view, this offers a serious problem as to reliability of the sintered material.
We have recognized that a main cause of such unsound sintering is the existence of agglomerates of boron in the green compacts or moldings, and we have confirmed that it is practically impossible to completely disintegrate and disperse some agglomerates existing in the initially added boron powder even by continuing ball-milling of the wet raw material for several days.